Sexual dimorphism of body and relative head sizes in neonatal common garter snakes

Morphological and behavioral differences between sexes are commonplace throughout the animal kingdom. Body size is one of the most obvious sex differences frequently found in snakes. However, the developmental origins of size differences in many species, including snakes, are not well known. We examined post-natal variation in sexual size dimorphism in garter snakes Thamnophis sirtalis . The weights, body and tail lengths, and head sizes of male and female neonates born to mothers collected from ecologically dissimilar habitats on Beaver Island, Lake Michigan were compared. Sexual size dimorphism was prominent. Overall, males had significantly longer bodies and tails than females. Females were significantly heavier and had larger heads than male snakes. Maternal site affected head but not body measurements, perhaps due to differences in prey availability. The body condition of maternal females predicted neonatal body length. Significant litter variation suggests heritable variation in morphological traits possibly correlated with feeding success and survival.     

Using time-dependent models to investigate body condition and growth rate of the giant gartersnake

Identifying links between phenotypic attributes and fitness is a primary goal of reproductive ecology. Differences in within-year patterns of body condition between sexes of gartersnakes in relation to reproduction and growth are not fully understood. We conducted an 11-year field study of body condition and growth rate of the giant gartersnake Thamnophis gigas across 13 study areas in the Central Valley of California, USA. We developed a priori mixed effects models of body condition index (BCI), which included covariates of time, sex and snout–vent length and reported the best-approximating models using an information theoretic approach. Also, we developed models of growth rate index (GRI) using covariates of sex and periods based on reproductive behavior. The largest difference in BCI between sexes, as predicted by a non-linear (cubic) time model, occurred during the mating period when female body condition (0.014±0.001  se ) was substantially greater than males (−0.027±0.002  se ). Males likely allocated energy to search for mates, while females likely stored energy for embryonic development. We also provided evidence that males use more body energy reserves than females during hibernation, perhaps because of different body temperatures between sexes. We found GRI of male snakes was substantially lower during the mating period than during a non-mating period, which indicated that a trade-off existed between searching for mates and growth. These findings contribute to our understanding of snake ecology in a Mediterranean climate.     

Temperature‐induced multi‐species cohort effects in sympatric snakes

In reptiles, reproductive maturity is often determined by size rather than age. Consequently, growth early in life may influence population dynamics through effects on generation time and survival to reproduction. Because reproductive phenology and pre‐ and post‐natal growth are temperature dependent, environmental conditions may induce multi‐species cohort effects on body size in sympatric reptiles. I present evidence of this using 10 years of neonatal size data for three sympatric viviparous snakes, Dekay''s Brown snakes (Storeria dekayi), Red‐bellied Snakes (S. occipitomaculata), and Common Garter snakes (Thamnophis sirtalis). End‐of‐season neonatal size varied in parallel across species such that snout–vent length was 36%–61% greater and mass was 65%–223% greater in years when gestating females could achieve higher April–May (vs. June–July or August–September) operative temperatures. Thus, temperature had a larger impact during follicular enlargement and ovulation than during gestation or post‐natal growth. Multi‐species cohort effects like these may affect population dynamics and the magnitude of these effects may increase with climate change.     

Population Structure of the Giant Garter Snake, Thamnophis gigas

The giant garter snake, Thamnophis gigas, is a threatened species endemic to California’s Central Valley. We tested the hypothesis that current watershed boundaries have caused genetic differentiation among populations of T. gigas. We sampled 14 populations throughout the current geographic range of T. gigas and amplified 859 bp from the mitochondrial gene ND4 and one nuclear microsatellite locus. DNA sequence variation from the mitochondrial gene indicates there is some genetic structuring of the populations, with high F_ST values and unique haplotypes occurring at high frequency in several populations. We found that clustering populations by watershed boundary results in significant between-region genetic variance for mtDNA. However, analysis of allele frequencies at the microsatellite locus NSU3 reveals very low F_ST values and little between-region variation in allele frequencies. The discordance found between mitochondrial and microsatellite data may be explained by aspects of molecular evolution and/or T. gigas life history characteristics. Differences in effective population size between mitochondrial and nuclear DNA, or male-biased gene flow, result in a lower migration rate of mitochondrial haplotypes relative to nuclear alleles. However, we cannot exclude homoplasy as one explanation for homogeneity found for the single microsatellite locus. The mitochondrial nucleotide sequence data supports conservation practices that identify separate management units for T. gigas.     

Morphological and ecological convergence in two natricine snakes

Similar morphologies between species may be due to shared ancestry or convergent evolution . Understanding instances of morphological and ecological convergence is central to evolutionary ecology because they help us understand the fit between organism and environment. Two species of stream-dwelling natricine snakes, Thamnophis rufipunctatus and Nerodia harteri present a model system for studying ecological and morphological convergence and adaptation. The species are allopatric and both live in shallow riffles in streams and forage visually for fish. We studied morphological similarity, trait evolution and functional significance of ecologically relevant traits in these and related species, and used mitochondrial DNA sequences for the ND4 gene to estimate their phylogenetic relationships. Character mapping of head length and head width supported the hypothesis of independent evolution of head shape in T .  rufipunctatus and N .  harteri . The elongate snout is a derived trait in these two taxa that is associated with reduced hydrodynamic drag on the snakes' heads when in a swift current, compared to other species with the ancestral blunt snout. We hypothesize that lower hydrodynamic drag facilitates prey capture success in these species that are known to forage by holding their position in currents and striking at fish prey. The elongate snout morphology has also resulted in a diminished binocular vision field in these snakes, contrary to the hypothesis that visually orientated snakes should exhibit relatively greater binocular vision. Convergent evolution of the long snout and reduced hydrodynamic drag in T. rufipunctatus and N. harteri are consistent with the hypothesis that the long snout is an adaptation to foraging in a swift current.  © 2005 The Linnean Society of London, Biological Journal of the Linnean Society , 2005, 85 , 363–371.     

Density‐Dependent Foraging and Interference Competition by Common Gartersnakes are Temperature Dependent

The ideal free distribution (IFD) predicts that optimal foragers will select foraging patches to maximize food rewards and that groups of foragers should thus be distributed between food patches in proportion to the availability of food in those patches. Because many of the underlying mechanisms of foraging are temperature dependent in ectotherms, the distribution of ectothermic foragers between food patches may similarly depend on temperature because the difference in fitness rewards between these patches may change with temperature. We tested the hypothesis that the distribution of Common Gartersnakes (Thamnophis sirtalis) between food patches can be explained by an IFD, but that conformance to an IFD weakens as temperature departs from the optimal temperature because fitness rewards, interference competition and the number of individuals foraging are highest at the optimal temperature. First, we determined the optimal temperature for foraging. Second, we examined group foraging at three temperatures and three density treatments. Search time was optimized at 27°C, handling time at 29°C and digestion time at 32°C. Gartersnakes did not match an IFD at any temperature, but their distribution did change with temperature: snakes at 20°C and at 30°C selected both food patches equally, while snakes at 25°C selected the low food patch more at low density and the high food patch more at high density. Food consumption and competition increased with temperature, and handling time decreased with temperature. Temperature therefore had a strong impact on foraging, but did not affect the IFD. Future work should examine temperature‐dependent foraging in ectotherms that are known to match an IFD.     

Responses of grassland snakes to tallgrass prairie restoration

Understanding faunal responses to habitat restoration is important in assessing restoration success. We investigated occupancy and abundance of snakes at Nachusa Grasslands, a large‐scale grassland restoration in the midwestern United States. Using artificial cover objects, we sampled within a chronosequence of 12 units converted from row‐crop agriculture 2–25 years before the start of our study. Recaptures of marked snakes revealed that movement distances differed among species in accordance with differences in body size, being least in Dekay's Brownsnakes, intermediate in the Plains Gartersnakes and Common Gartersnakes, and greatest in Eastern Foxsnakes. Consistent with this result, occupancy increased with restoration age in Dekay's Brownsnakes but was unrelated to restoration age in the three larger, more mobile species. Similarly, abundance increased with restoration age in Dekay's Brownsnake but was unrelated to restoration age in other species. The Smooth Greensnake, another small‐bodied snake with limited mobility, and an Illinois species of greatest conservation need, was not detected at Nachusa Grasslands. Given detection probabilities observed during a parallel study at a nearby large grassland‐dominated preserve, we infer that the Smooth Greensnake is truly absent from Nachusa Grasslands. Taken together, our results demonstrate that establishment of faunal components following restoration may be time‐dependent with more sedentary species colonizing only slowly (e.g. Dekay's Brownsnakes) or not at all (e.g. Smooth Greensnakes). These results emphasize the need to clearly identify faunal restoration goals and the means to achieve them.     

Gene flow and melanism in garter snakes revisited: a comparison of molecular markers and island vs. coalescent models

Within populations, the stochastic effect of genetic drift and deterministic effect of natural selection are potentially weakened or altered by gene flow among populations. The influence of gene flow on Lake Erie populations of the common garter snake has been of particular interest because of a discontinuous colour pattern polymorphism (striped vs. melanistic) that is a target of natural selection. We reassessed the relative contributions of gene flow and genetic drift using genetic data and population size estimates. We compared all combinations of two marker systems and two analytical approaches to the estimation of gene flow rates: allozymes (data previously published), microsatellite DNA (new data), the island model ( F _ST-based approach), and a coalescence-based approach. For the coalescence approach, mutation rates and sampling effects were also investigated. While the two markers produced similar results, gene flow based on F _ST was considerably higher (Nm > 4) than that from the coalescence-based method (Nm < 1). Estimates of gene flow are likely to be inflated by lack of migration-drift equilibrium and changing population size. Potentially low rates of gene flow (Nm < 1), small population size at some sites, and positive correlations of number of microsatellite DNA alleles and island size and between M , mean ratio of number of alleles to range in allele size, and island size suggest that in addition to selection, random genetic drift may influence colour pattern frequencies. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 79, 389–399.